The present invention relates to an ignition timing control system for an engine, and more particularly to the timing control system for shifting from a fixed ignition timing after complete firing to an ordinary ignition timing control in dependency on an engine temperature.
Heretofore, for the ignition timing control system of this kind, there is an angular control method to detect projections or slits provided on a crank rotor rotating in synchronism with a crank shaft to messure an ignition timing as disclosed in e.g., Japanese Patent Application Laid-Open 61-96181. In addition, there is a time control method to detect passing time between the projections or slits provided on the crank rotor at predetermined intervals to measure an ignition timing as disclosed in Japanese Patent Laid-Open 60-47877, etc.
Meanwhile, because an engine speed at the time of cranking is unstable, many systems have a measure to fix the ignition timing at position before top dead center (BTDC) 10.degree. at the time of cranking and then to advance an ignition angle after starting the engine to shift to an ordinary ignition timing. Generally, a timing for shifting to such an ordinary ignition timing is uniformly switched to the ordinary ignition timing when a starter switch is switched from an ON to an OFF state in dependency on the engine speed.
In an ordinary operating state where the engine speed is stable, the time control method is more advantageous than the angle control method in various aspects, i.e. fast computing speed and simple structure. However, for an unstable initial or start-up time period immediately after starting, it is difficult to precisely detect changes in the engine speed.
Namely, as shown by fixed ignition time period at the time of cranking in FIG. 1 and the ignition timing control immediately after starting in FIG. 2, projections 1a and 1b are formed at an outer periphery of a crank rotor 1, e.g., at positions of BTDC 10.degree. and BTDC 100.degree.. At the time of cranking, when a crank pulse produced in response to detection of the projection 1a is output for the fixed ignition timing, an ignition signal is output to an ignition drive means (not shown) to spark an ignition plug (state of FIG. 1).
On the other hand, when the starter switch is turned OFF after complete firing, or when the engine speed rises to a predetermined value, the ignition timing control is switched to the ordinary ignition timing control. First, an angular velocity is calculated from a time period .alpha. from the time when the projection 1a is detected to the time when the projection 1b is detected to convert an ignition angle set depending upon the operating state to an ignition timing in accordance with a calculated angular velocity, thus to measure the ignition timing using the time when the projection 1b is detected as a reference time point. When the time reaches a predetermined ignition timing (BTDC 20.degree. in FIG. 2), an ignition signal is output.
However, the combustion characteristics generally vary in dependency on a combustion temperature. For example, firing at the initial time of complete firing at a high engine temperature is relatively stable. Accordingly, the shift of the ignition timing is relatively fast from the fixed position and to the ordinary ignition timing position and permits a smooth start-up characteristic. On the other hand, where the engine temperature is low such as in a cold starting, combustion becomes unstable also after complete firing. Particularly, in the case of an extremely low engine speed immediately after starting the engine, an interval of the time period .alpha. is prolonged. When the engine speed for this time period varies to much degree, even if the ignition timing is at BTDC 20.degree. as shown in FIG. 2, an actual ignition angle may be excessively advanced to an extent of BTDC 30.degree..
As a result, when the ignition timing is suddenly advanced from the fixed ignition timing when starting the engine in the cold state, the engine speed is not smoothly increased. Consequently, engine stall would occur, thus making it difficult to obtain a satisfactory starting performance.
In addition, when switching timing of such an ignition timing is set in correspondence with the cold state, the ignition timing control at a low engine speed at a high engine temperature is not suitably conducted, resulting in the problem that a satisfactory starting or restarting performance cannot be obtained.